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Article Cite This: ACS Omega 2019, 4, 10328−10335
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Suppressing the Shuttle Effect in Lithium−Sulfur Batteries by a UiO66-Modified Polypropylene Separator Yanpeng Fan, Zhihui Niu, Fei Zhang, Rui Zhang, Yu Zhao,* and Guang Lu* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Renai Road, Suzhou, Jiangsu 215123, P. R. China
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S Supporting Information *
ABSTRACT: The lithium−sulfur battery is one of the most promising battery technologies with high energy density that exceeds the presently commercialized ones. The shuttle effect caused by the migration of soluble polysulfides to the lithium anode is known as one of the crucial issues that prevent the Li−S batteries from practical application. Modification of the separator is regarded as a convenient yet efficient strategy to alleviate the shuttle effect. In this report, we use a thermally stable and chemically robust metal−organic framework (MOF), UiO-66, as a physical and chemical barrier for soluble polysulfides to functionalize the commercial polypropylene separator. The Li−S cell assembled with such a separator shows a significantly improved cycling stability with an average specific capacity of ca. 720 mA h g−1 at a current rate of 0.5 C for 500 cycles. Experimental and theoretical investigations indicate that the cell performance enhancement results from the physical restriction of the MOF barrier layer and strong chemical interaction between UiO-66 and polysulfides. The excellent thermal stability and chemical robustness (in acid/ alkali solutions, conventional organic solvents, and polysulfide electrolytes) of UiO-66 make it highly competitive among various materials developed for separator modification in Li−S batteries.
1. INTRODUCTION The lithium−sulfur battery has been considered as one of the most promising battery technologies because of its high energy density and the low-cost and high abundant sulfur element.1,2 However, there are still some crucial challenges toward the practical application of lithium−sulfur batteries, such as the shuttle effect of highly soluble polysulfide intermediates, which leads to internal short circuit inside the cell.3−6 Many strategies have been proposed to alleviate the shuttle effect, for example, physical and/or chemical confinement of the soluble polysulfide intermediates in the cathode.7−14 Alternatively, functionalizing the cell separator with an additional barrier layer also provides a straightforward approach.15−21 In consideration of the compatibility of lithium−sulfur batteries with the existing battery technologies, it is preferred to use components well-developed for lithium-ion batteries such as the commercialized polypropylene (PP) or polyethylene separators. Such separators are advantageous in cost, mechanical strength, chemical stability, and electrochemical stability. However, they are incapable of restraining polysulfide diffusion. Accordingly, some functional materials are implanted on the commercialized separators as the blocking interlayer to prevent polysulfide migration from the cathode to the anode while maintaining the capability of ion conductivity.22−24 Metal−organic frameworks (MOFs) are a class of porous crystalline materials composed of metal ions (or clusters) linked by organic ligands.25−27 Because of their large surface areas, regular pore sizes, and tailorable chemistry, MOF © 2019 American Chemical Society
materials have shown great promise for a variety of applications.28−34 Given their uniform (sub-) nanometersized pore structures, MOFs would be promising candidate materials to construct separators capable of alleviating the shuttle effect.35−40 Three strategies have been developed for the preparation of MOF-based/-derived cell separators. For example, He et al. used HKUST-1 nanoparticles as assembly units and polyvinylidene difluoride (PVDF)−hexafluoropropylene as a binder to prepare the separator,41 which served as a good physical barrier to confine the polysulfide intermediates in the cathode side and led to a stable Li plating/stripping even at high current densities. He et al. reported the in situ growth of ZIF-67 and its conversion into hollow Co9S8 arrays on a PP separator,42 which functioned as an efficient polysulfide barrier for high-performance lithium−sulfur batteries. Wu et al. directly casted ZIF-8/carbon nanotube/PVDF slurry onto one side of a PP separator and improved significantly the reversible capacity and cycling stability of lithium−sulfur batteries.43 Until now, however, little attention has been paid on the stability of MOFs on exposure to the electrolyte environment. In fact, the mismatched standard redox potentials of the transition metals in MOFs and polysulfides/ lithium might raise the risk of structure collapse of the framework materials.44 Received: March 30, 2019 Accepted: June 3, 2019 Published: June 13, 2019 10328
DOI: 10.1021/acsomega.9b00884 ACS Omega 2019, 4, 10328−10335
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Article
Figure 1. Characterization of the as-prepared UiO-66 crystals. (a) XRD patterns of UiO-66 and crystal structures showing the Zr6-octahedron cluster, octahedron cage, and tetrahedral cage in the UiO-66 frameworks. (b) Representative SEM image of the as-prepared UiO-66 crystals and elemental mapping of C, O, N, and Zr elements. (c) TGA trace (normalized with the final weight to 100%) measured at a ramping rate of 10 °C/ min in air. (d) N2-sorption isotherms (upper panel) and pore size distribution (lower panel) of the evacuated UiO-66 crystals.
Figure 2. Characterization of the UiO−PP separator. (a) Surface (upper) and cross-sectional (lower) structure of the UiO−PP separator. (b) Diffusion of the polysulfides in the visualized bottle using UiO−PP and PP separators. (c) Nyquist plots (upper) and corresponding equivalent circuit (lower) of a Li−S cell using UiO−PP and PP separators.
greatly facilitates the alleviation of shuttle effect and thus contributes to lithium−sulfur cells with improved stability and cyclability.
In addition, most investigated MOFs are very sensitive to moisture or the acidity of the electrolyte, which adds on additional restriction to the electrolyte choice.45 Therefore, it is crucial to search for proper MOF materials that can effectively alleviate the shuttle effect through physical and/or chemical interactions at the molecular level while maintaining good structural stability.46,47 Herein, we report the UiO-66-modified PP separator that enables a significant improvement in cyclability of lithium−sulfur batteries. The UiO−PP separator was prepared by casting the slurry composed of MOF crystals, Super P carbon, and PVDF in 1-methyl-2-pyrrolidinone on a commercial PP separator. Electrochemical performance study indicates that the UiO−PP separator can significantly improve the cycling stability of the assembled cells over the conventional PP separator, with an average specific capacity of ca. 720 mA h g−1 at a current rate of 0.5 C for 500 cycles. Experimental results and first-principle calculations suggest that the chemical adsorption of polysulfides on UiO-66 frameworks combined with the physical restriction effect of the UiO−PP separator
2. RESULTS AND DISCUSSION UiO-66 crystals were synthesized via the base/acid comodulated method.49 This controlled synthesis strategy allows for the large-scale production of UiO-66 octahedral microcrystals with the uniform but tunable sizes. X-ray diffraction (XRD) analysis indicated that the as-synthesized product was crystalline and displayed a diffraction pattern (Figure 1a) that is identical to that of UiO-66.48 The crystal structure was built with hexanuclear Zr6-octahedron clusters connected by terephthalic acid, featuring regular octahedral cages (11 Å) and tetrahedral ones (9 Å) connected by the 0.6 nm triangular window. Scanning electron microscopy (SEM, Figure 1b) revealed that the product consisted of octahedral crystals with an average size of ca. 400 nm and a relatively narrow size distribution. Energy-dispersive X-ray microanalysis confirmed 10329
DOI: 10.1021/acsomega.9b00884 ACS Omega 2019, 4, 10328−10335
ACS Omega
Article
Figure 3. Li−S cell performance using UiO−PP, SiO2−PP, and PP as separators. (a) CV profiles of the Li−S cells in the potential range of 1.5−3.0 V (vs Li+/Li) at a sweeping rate of 0.2 mV s−1. (b) Initial galvanostatic charge/discharge profiles at a current rate of 0.5 C. (c) Galvanostatic specific charge/discharge capacity (Cs) and corresponding Coulombic efficiency (ηCE) of the Li−S cells. (d) Rate performance and corresponding CE of the Li−S cells. In all panels, the colors of red, blue, and black are assigned to the cells using UiO−PP, SiO2−PP, and PP as separators, respectively.
separator with UiO-66 can restrain effectively the polysulfide diffusion. To verify whether such a capability brought down the Li+ion conductivity across the separator in a Li−S cell, electrochemical impedance spectroscopy (EIS) was used to study the internal resistance of a CR2032 coin cell consisting of 1 M LiTFSI in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) as the electrolyte, S/Super P carbon as the cathode, Li metal as the anode, and UiO−PP or PP as the separator. The Nyquist plots and Nyquist equivalent circuit are shown in Figure 2c. In the equivalent circuit, Re, Rct, and Rf stand for the Ohmic resistances of the electrolyte, charge transfer, and the film electrode, respectively. CPEct and CPEf represent the constant-phase element. Zw is the Warburg impedance of the process of Li+ ions diffusing into the electrolyte. Both cells exhibited the similar Ohmic resistances of the electrolyte (6.2 Ω for the UiO−PP separator and 6.9 Ω for the PP separator, respectively). It is known that the soluble polysulfides can shuttle back and forth freely through the channels of the PP separator, causing the shuttle effect in a Li−S cell. However, for the UiO−PP separator, these channels were blocked, benefitting to suppress the migration of polysulfides to the Li anode. Therefore, the cell with a UiO−PP separator exhibited lower charge transfer (27.1 Ω) and film electrode resistances (15.2 Ω) than the PP separator counterpart (31.8 and 38.4 Ω, respectively). Besides, the open-circuit voltages of Li−S cells using the UiO−PP separator exhibited an antiself-discharge feature as shown in Figure S2 (Supporting Information). It was observed that the open-circuit voltage of a normal cell only with a PP separator remained at ca. 2.35 V versus Li+/Li with gradual decay in the measured 180 h, which indicates an immediate spontaneous reduction of sulfur into high-order polysulfides. Whereas this self-discharging was inhibited by the incorporation of the UiO−PP separator, of which the opencircuit voltage remained stable at ca. 2.7 V versus Li+/Li for more than 150 h because of the prohibition of soluble sulfur emigration into the anode. Nevertheless, cyclic voltammetry (CV) tests with different sweep rates (Figure S3, Supporting Information) indicated that the lithium-ion diffusion coef-
the existence of C, O, and Zr and residual dimethylformamide (DMF) molecules in the as-prepared UiO-66 crystals.50 Thermogravimetric analysis (TGA) was carried out to evaluate the thermal stability of the as-prepared UiO-66 sample. The TGA traces showed weight losses in the temperature ranges of 25−230, 230−390, and 390−550 °C, corresponding to the weight loss from volatilization of solvent molecules, the dehydroxylation and elimination of monodentate modulators, and the decomposition of 1,4-benzenedicarboxylic linkers, respectively (Figure 1c).48,51 The porosity was investigated by N2-sorption measurement at 77 K. As shown in Figure 1d, the evacuated UiO-66 sample exhibited the type I isotherms with a rapid increase in N2 uptake at low relative pressure (
